Radiotherapy has been used for treating cancer in the human body since early 1900. Even though radiation of cancer tumours is known to be efficient, mortality rate for many cancers remained virtually unchanged for a long time. The major cause to this has been the inability to control the primary tumour or the occurrence of metastases. Only by improving the local control may the treatment be more effective.
Therefor the radiotherapy methods and devices have developed in the that sense the energy levels of the radiation accelerators have increased, introduction of collimators in order to shield off and direct the radiation to desired areas, to detect the size and location of the tumour in the patient and to calculate the required dose in dependence of the detection.
Further measures for improving the control are In Vivo dosimetry and Portal image systems. In In Vivo dosimetry the radiated dose is measured outside the human body to verify the delivered dose during treatment. The aim of the In Vivo dosimetry is in most cases to predict the dose inside the body while measuring the dose on the outside. Certain corrections need therefor be made of the detected signal. Conventionally In Vivo dosimetry is used in TBI (Total Body Irradiation) and to verify fraction doses in fractional treatment. This is often done during the first fraction and maybe a few subsequent fractions, i.e. every sixth fraction. The aim is then to verify the predicted dose to the patient and to reduce the number of errors, systematic and/or stochastic, and predict the accumulated dose to the patient if it is applied at each treatment fraction and thereby increase the outcome of the treatment.
The main reasons why general In Vivo dosimetry is not performed frequently are that the detector shields parts of the treatment area of the patient, thereby reducing the dose and in turn the tumour control, and the time and patient flow since it is rather time consuming to apply the detectors onto the patient for each fraction.
The main purpose for the use of Portal Imaging is to verify the position of the patient, either during treatment or afterwards. Traditionally a radiographic film has been placed where the radiation beam exits the patient for exposure during one field at one fraction. The image on the film is then used to verify the position of the patient compared to the treatment plan. In recent years it has become common to use Electronic Portal Imaging (EPI) devices to achieve instant information of the patient positioning.
One problem with the method of measuring only at the first fraction, or at a few fractions, and then rely on those values at the other fractions is that the accelerator of the radiation device is somewhat unstable, i e the output from the accelerator may vary in time due to many factors. Thus, it is not certain that the dose level delivered and measured is the same throughout all fractions, providing uncertainty in the treatment.
Because of that, Portal imaging and in particular EPI has been proposed for In Vivo dosimetry. However there are important limitations using those devices for dosimetry as regards to e g energy dependency, dose rate dependency, dependency in distance to patient etc., primarily since these devices were developed for image capturing and processing and not for dose level detection and measurement.
A few attempts have been made to explore if the optical density of a portal image film could be used as a measure of the relative exit dose. One such research report is disclosed in the publication Radiotherapy and Oncology 29 (1993) 336-340, by C. Fiorino et al. In the report different phantoms have been radiated, the exit dose measured by an ionization chamber and the dose captured on the film. The conclusion of the research is that a good agreement has been found between measured and calculated dose profiles on one hand and optical density profiles on the film on the other hand for different phantoms.
One major problem with the use of a portal imaging device as a measure of the relative dose as described above, is to implement it clinically. The exit dose when treating a human body is very different from radiating a phantom, so corrections have to be calculated in order for the optical density profiles obtained during treatment to correspond to those obtained with the phantom. The uncertainty of the corrections, depending very much on the anatomy of the actual patient and the location of the beam during treatment, implies that there is a large amount of uncertainty in the values obtained from the portal image device.